The present invention concerns a hand held piece for dental or surgical use, comprising a hollow shaft, associated with rotational drive means and in which there are mounted a clamp for gripping the shank of a removable tool, a loosening sleeve and a sliding pusher placed in a rear end of the hollow shaft and controlled by a manual button to move the clamp and the loosening sleeve closer to each other, the clamp including an annular part and jaws arranged for gripping the shank of the tool by resilience in a working position, the jaws extending longitudinally from the annular part towards the loosening sleeve, which comprises oblique external surfaces able to abut against the free ends of the jaws to move them apart.
In most cases, hand held pieces of this type are driven by an air turbine and rotate at very high speeds, of up to more than 400,000 revolutions per minute, which imposes high requirements on the centring of the tool in relation to the axis of rotation. In order to ensure sufficient holding of the tool while working, the resilient clamp has to resist a standard pull strength of at least 22 N on the tool. Moreover, since the operator has to change tools frequently, this operation must be easy and cause the least possible wear.
In a typical hand held dental piece of this type, as disclosed for example in EP Patent Nos. 273 259 and 505 599, the clamp is fixed in the hollow shaft, its annular part is on the tool side, i.e. at the front end of the shaft, and its jaws extend axially backwards, where the loosening sleeve can slide into the hollow shaft via the effect of a manual pusher connected to the mobile back cover of the hand held piece. The loosening sleeve comprises at the front an external cone provided with support surfaces having a larger angle of aperture than the opposite surfaces of the inner cone of the clamp jaws, such that the contact between the two cones occurs along a line which does not change place on each jaw during the travel of the pusher.
With the design according to EP Patent No. 273 259, the annular part of the clamp centres the tool shank at the front end of the rotating part, whereas the back end of the shank is centred inside the loosening sleeve. Since this part slides, it necessarily has a slight radial play and centring is therefore not excellent. Moreover, manufacturing and balancing the piece that forms both the clamp and the guide bush are difficult and expensive operations. Another drawback of this arrangement lies in the total length of the assembly comprising the clamp, said sleeve and the manual pusher. The length means that the head of the hand held piece must have a certain height, which is inconvenient since this is the part that the dentist has to put in the patient's mouth.
EP Patent No. 505 599 proposes improving this design by means of a guide bush at the front and a fixed guide arranged at the back end of the hollow shaft, thus centring the tool shank better and comprising, inside, a fixed stop member for the tool shank, but the length of the assembly is thereby increased. Further, this arrangement has another drawback, in that the loosening sleeve can become blocked by friction in the open clamp, and it is unable to be unblocked by inserting the tool shank, since the axial stop member is not on the sleeve.